Fixed script prefixes in absolute URLs

src/pretix/helpers/urls.py

@@ -1,5 +1,9 @@
+from urllib.parse import urljoin
 from django.conf import settings
+from django.core.urlresolvers import reverse
 
 
-def build_absolute_uri(url):
-    return settings.SITE_URL + url
+def build_absolute_uri(urlname, args=None, kwargs=None):
+    # Pass prefix='' as a possible SCRIPT_PREFIX (if pretix runs in a subdirectory)
+    # is included in SITE_URL _and_ is added by reverse.
+    return urljoin(settings.SITE_URL, reverse(urlname, args, kwargs, prefix=''))